A bipolar junction transistor (BJT) comprises three adjacent doped semiconductor regions or layers having an NPN or PNP doping configuration. A middle region forms a base and two end regions form an emitter and a collector. Typically, the emitter has a higher dopant concentration than the base and the collector, and the base has a higher dopant concentration than the collector. Generally, the BJT can be operated as an amplifier (for example, to amplify an input signal supplied between the base and the emitter, with the output signal appearing across the emitter/collector) or as a switch (for example, an input signal applied across the base/emitter switches the emitter/collector circuit to an opened or a closed (i.e., short-circuited) state. In operation, the emitter/base pn junction is forward biased and the collector/base pn junction is reverse biased.
There are several known semiconductor fabrication processes for forming the three doped layers of a bipolar junction transistor and several transistor architectures can be formed according to such processes. The simplest structure is a planar architecture with the stacked NPN or PNP regions formed by successive dopant implants into a semiconductor substrate.
A metal-oxide field effect transistor (MOSFET) differs in structure and operation from a BJT. The MOSFET comprises source and drain doped regions formed in a tub or well having an opposite doping type. A voltage applied to a gate disposed above the well between the source and drain inverts the conductivity of a channel region between the source and the drain, permitting current flow through the channel.
In certain MOSFET devices it may be desired to bias a p-doped tub of an NMOSFET separately from the substrate in which the tub is formed. This bias arrangement requires complete isolation of the entire p-tub from the substrate. One common isolation technique comprises junction isolation, for example, a triple well process wherein an implant step dopes n-type a region of the silicon substrate immediately below the p-tub. A subsequent n-type tub implant forms a ring of n-type material around the periphery of the p-tub, extending downwardly to connect with the n-type triple well to isolate the p-tub from the substrate. The triple well and tub implant steps are performed prior to forming the MOSFET gate and its associated components.
Most integrated circuits for processing digital signals comprise a plurality of complimentary metal-oxide semiconductor field effect transistors (CMOSFETS) each of which further comprises an arrangement of an NMOSFET formed in a p-type substrate well and a PMOSFET formed in an n-type well. Certain of these integrated circuits also include bipolar junction transistors and thus are referred to as BiCMOS circuits. CMOS devices consume less power and occupy less integrated circuit area than the BJT. However, BJT's generally exhibit a higher switching speed.
It is known that there is a maximum reverse voltage that can be applied across any two junctions of the BJT before the junction breaks down. Break down current flow through the reverse-biased junction can generate excess heat and may destroy the junction and thus the transistor. The breakdown voltage of a pn junction decreases as the doping level on either or both sides of the junction increases. Thus, to increase the breakdown voltage the doping level on one or both sides of the junction is reduced. The lower dopant density lowers the electric field intensity across the junction for a fixed applied voltage, or stated differently, raises the voltage at which junction breakdown occurs. However, there are known disadvantages to low-doped junctions, including a higher junction resistance and slower switching speed or response time to an input signal.
The two BJT breakdown characteristics of interest are BVCEO (collector-emitter breakdown voltage with the base open circuited) and BVCBO (collector-base junction breakdown voltage with the emitter open circuited). Since the collector current injected into the base region is multiplied by the transistor gain (β), these two breakdown characteristics are related byBVCEO/BVCBO≈(1/β)1/m where m is between about 3 and 6.
Cost considerations constrain BiCMOS fabricators to offer one or at most two BJT breakdown classes or types in an integrated circuit, i.e., a first type exhibiting a relatively high breakdown voltage (i.e., BVCEO and BVCBO) and a second type exhibiting a relatively low breakdown voltage. To form two BJT from each class in an integrated circuit, a first mask is employed to expose certain collector regions where a first dopant concentration is implanted using an ion dose with a first implant energy. A second mask exposes other collector regions for receiving a second implant ion dose at a second implant energy to form a second dopant concentration. Offering bipolar transistors with other (e.g., intermediate) breakdown voltages provides the circuit designer with additional design flexibility, but requires developing another mask and adding additional implant and mask process steps, thereby increasing fabrication costs. Thus providing additional transistor breakdown classes may not be justified by the extra cost of the masking and implant steps.
Recognizing these cost limitations, during integrated circuit design, BJT breakdown characteristics are selected based on perceived customer needs, and circuit designers are restricted to the selected BJT breakdown classes fabricated in the integrated circuit. This “one size fits all” compromise approach can cause difficulties in implementing a circuit with desired operational characteristics when the designer is restricted to only two breakdown values for the BJT's on the integrated circuit. It is thus desired to expand the number of NPN breakdown classes without increasing the fabrication cost.
Each integrated circuit foundry employs specific fabrication process parameters that are unique to the foundry, e.g., wafer routing, process checks, process recipes, process documentation, test conditions and specification limits. As a result, difficulties may be encountered in transferring fabrication of an integrated circuit product (including a BJT) from one foundry to another, as the receiving foundry may not possess the capability to manufacture the integrated circuit. To successfully transfer the product, the receiving foundry must match the process parameters of the transferring foundry, which can be a difficult and costly undertaking.
As related to fabrication of BJT's with different breakdown voltages, each foundry typically individually optimizes its NPN voltage breakdown offerings. Thus it is difficult to transfer a BJT product from one foundry to another. A foundry capable of producing several different BJT breakdown types is more likely to successfully fabricate a BiCMOS integrated circuit transferred from another fabrication foundry.